Chain getting caught on frame

[Slowroad]

A few months ago I bought a bike for commuting - it's great except that the chain occasionally gets caught between the smallest front cog and the frame, scratching off the paint and bringing me to a sudden halt - not good in traffic. It's not actually falling off the cog (I think I know how to sort that out) - it's apparently either jumping up or being pulled up behind the cog and getting stuck against the frame. This happens when changing down from the middle to the smallest cog - but only occasionally.
It was a mailorder bike from a good company but it's difficult for them to help via phone or email. I'm loath to take it to a bike shop straightaway - any suggestions?

[thirdcrank]

Your front mech should have two little screws which limit the movement of the cage - one for each direction. Without going out to look at mine, I think the low adjustmnet screw (which is the one you want) is labelled 'A' on others it's different and may not be labelled at all so you have to work out which is which.

Tighten the screw just a tad at a time till the chain stops coming off. Don't over-adjust or the cage will begin to scrape during normal riding and may even stop changing properly.

If your gears are Shimano have a look at http://techdocs.shimano.com/techdocs/index.jsp then navigate to your model of equipment for pukka instructions. (SI = sevice instructions, which is what you want here.)

Here's chapter and verse from the ver reliable SB http://www.sheldonbrown.com/derailer-adjustment.html

[squeaker]

Try adjusting the L screw on the front dérailleur.
HTH

[CREPELLO]

It's not actually falling off the cog (I think I know how to sort that out) - it's apparently either jumping up or being pulled up behind the cog...

Could there be some chain suck happening here?

[Si]

I agree with CREPELLO, sounds like chain suck rather than the limit screw being set wrong. Did you buy the bike new or second hand - if second hand it might have a worn ring and/or chain, although there are a number of explanations for chainsuck.

[slugbike]

Sounds like chain suck to me ... that instant sudden halt. I heard lots of reasons for why it happened and almost learnt to live with it... that feeling of stop pedaling instantly you could feel it happening. Then I changed my chain... and it stopped. The problem had been there almost from when I bought the bike from the shop and it had been returned a couple of times to the shop about it as well.

[meic]

Chain suck is something that I have learned to live with.
Traditional wisdom blames dirty chain and rings or a good chain on worn rings.
I have cured some chain suck by replacing worn rings but mostly it occurs on limited occasions and you just learn to deal with it.
I suspect that I am prone to it because I try and have an excessive gear range. This means a lot of chain for the rear deraileur to try and pull in whenever I change front rings.

[Slowroad]

Wow! So many comments so soon!
It does sound like the rather unpleasantly-named 'chain suck' - it's almost as if the chain gets sucked up then gets caught.
It's a new bike, but I've ridden it quite a lot, and the chain was a bit dirty so I've cleaned it. I've also noticed that there is very little space between the cogs and the frame - far less than my previous bikes. It's also the first bike I've had which changes gears with a click (don't know proper terminology!) So it may be that it's my riding which is partly at fault - I'm used to changing gears more slowly, and I'm used to bikes where the chain can mess about more without getting caught up!
So I'll be changing gear more carefully and buying a tin of hammerite...
Any further thoughts most welcome.

[Deckie]

I got a little gizmo that fits round the down tube below the fron derailleure level with the inner ring. It's plastic & looks like an inverted hook. The idea is that if the shifter pushes the chain too far, or it tries to jump over the inner ring it slips back on & can't (unless you're really unlucky) slip down between the inner ring & down tube.

Pro racers use them too even though they are famously scared of adding anything onto the bike that doesn't make them faster due to the weight.

I got it at the LBS, but can't remember the make or even what they called it properly.